xdg-shell: fix a typo

Signed-off-by: Sebastian Krzyszkowiak <dos@dosowisko.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>

stable/xdg-shell/xdg-shell.xml

@@ -101,7 +101,7 @@
       <description summary="check if the client is alive">
 	The ping event asks the client if it's still alive. Pass the
 	serial specified in the event back to the compositor by sending
-	a "pong" request back with the specified serial. See xdg_wm_base.ping.
+	a "pong" request back with the specified serial. See xdg_wm_base.pong.
 
 	Compositors can use this to determine if the client is still
 	alive. It's unspecified what will happen if the client doesn't